1. Field of the Invention
The invention relates to a method and system for reducing the number of false-positive detections of abnormalities in X-ray images by using digital image analysis techniques with a computerized scheme to analyze pulmonary nodule candidates or mammographic masses and microcalcifications for possible abnormality conditions indicative of disease.
2. Discussion of the Background
It is known that radiologists can fail to diagnose pulmonary nodules from chest radiographs in as many as 30% of positive cases. In addition, in a study by Muhm et al., Radiology, Vol. 148, pp. 609-615 (1983), it was reported that 90% of peripheral lung cancers were visible in retrospect on previous films. In order to alert the radiologist to locations of possible nodules or breast masses and microcalcifications, and to reduce the number of false-negatives diagnoses, the present inventors have developed a computer-aided diagnosis scheme for the detection of lung nodules in digital chest images and mammographic masses and microcalcifications. Such a computerized diagnosis scheme is disclosed in U.S. Pat. No. 4,907,156 to Doi et al, incorporated herein by reference.
The method and system are based on a difference-image approach in which a nodule-suppressed image is subtracted from a nodule-enhanced image in order to remove the complex background in a chest image, and thus enhance the conspicuity of nodules. After a difference image is obtained, feature-extraction techniques are employed which involve the size, contrast and shape of the nodule candidates. Using 60 clinical chest images, the sensitivity obtained with this previous method was approximately 70% at a false-positive rate of seven or eight per image. It is apparent that a practical limitation of this previous method is the existence of these false-positive detections which can lead to incorrect diagnoses by radiologists interpreting the computer generated data, i.e., a diagnosis of abnormality for an actually disease-free lung or breast.
In the prior art, a number of false-positives obtained with this computer scheme usually occurred at rib crossings, and at rib-and-vessel crossings. In addition, some end-on vessels and also aggregates of vessels produced false-positives. These normal anatomical background features will often appear similar to actual nodules (dense clusters of diseased cells) due to the shadows they produce on digitized chest X-rays using various gray levels. The different gray levels represent the level of opacity of the individual anatomical features. The image enhancement technique referred to above increases the sensitivity of the detection process, thus generating a higher number of possible actual nodule candidates. Such a technique is important so as not to miss any actual nodules which may be of relatively small size. In other words, the increased sensitivity reduces the number of false-negatives. However, the increased sensitivity tends to increase the number of false-positive detections as well by erroneously detecting normal background structures, commonly referred to as artifacts. Thus, one of the main concerns of the present invention is to be able to reduce the number of false-positive detections, while still maintaining the increased sensitivity.